Recently, people are more concerned with environmental problems and depletion of natural sources and thereby, interest in solar cells as an alternative energy source which does not cause environmental pollution is growing. Solar cells are classified into silicon solar cells, thin film-type compound solar cells, layered-type solar cells and the like. Among these solar cells, silicon semiconductor solar cells have been studied the most widely.
However, since silicon solar cells are indirect transition semiconductors, light absorption coefficients of which are lower than those of direct transition semiconductors, silicon solar cells cannot efficiently absorb photons, compared to direct transition semiconductors. Thus, silicon solar cells require a wider space charge region than direct transition semiconductors. In addition, to prevent recombination of electrons and holes, which are generated due to extended carrier life, in a space charge region, high purity Si is essentially required and, as such, high-priced, high level and complex process technologies, which consist of plural steps, and a high vacuum thin film process are required. When high purity single crystal Si is used, solar cells exhibit high efficiency. However, production costs of such a high purity single crystal Si are high and thereby, to reduce production costs, polycrystalline Si or amorphous Si, which exhibit low efficiency, are used. Since such polycrystalline Si or amorphous Si has low photoelectric conversion efficiency, when used for a long time, a problem such as deterioration may occur.
Therefore, to improve the problems of silicon solar cells, thin film type compound solar cells are recently studied and developed.
Among thin film type compound semiconductors, Cu(In1-xGax)(SeyS1-y) (CI(G)S), which is a Group I-III-VI compound included in ternary compounds, has a direct transition type energy band gap of 1 eV or more and high light absorption coefficient. In addition, the Cu(In1-xGax)(SeyS1-y) (CI(G)S) is very stable electro-optically. Thus, the Cu(In1-xGax)(SeyS1-y) (CI(G)S) is an ideal material for a light absorption layer of solar cells.
CI(G)S based solar cells are manufactured by forming a light absorption layer having a thickness of several microns. As methods of manufacturing a light absorption layer, there are an evaporation method which does not require a precursor, and sputtering and electrodeposition methods which form a CI(G)S thin film through thermal treatment after forming a thin film with a precursor. In addition, an ink coating method, in which thermal treatment is conducted after coating a precursor material under non-vacuum, was recently introduced. By using the ink coating method among the above methods, process costs may be reduced and a large area may be manufactured homogenously. Thus, research into the ink coating method is actively being carried out. As a precursor in the ink coating method, a variety of compound or metal types such as metal chalcogenide compounds, bimetallic metal particles, metal salts, metal oxides, or the like may be used.
In particular, when a metal chalcogenide compound is used as a precursor, a Cu—Se compound and In—Se compound are mixed or CuInSe2 particles are synthesized. When particles are mixed, a partially heterogeneous coating layer may be produced. When CuInSe2 is used, a long time is required for particle growth.
Meanwhile, since bimetallic metal particles are synthesized with Cu—In alloy, partial heterogeneity may be solved, and reaction time is reduced due to fast particle growth. However, in a selenium (Se) or sulfur (S) atmosphere, a layer with a partial Se or S deficit may be formed. In addition, when a metal salt is coated, a coating layer having high layer density may be produced, whereas due to anions included in a salt, a layer is damaged or organic residues may be formed.
Therefore, there is a high need to develop a technology for precursor nano particles which may form a highly efficient light absorption layer, a coating property of which is improved and thereby layer density of which is increased.